A cutting insert is firmly coupled to a body of a tool in order to provide the tool with cutting edges. FIG. 1 shows a cutting insert fixed to a milling cutter using a screw. A cutting insert may also be coupled to the body of a tool by other means such as wedge fixing means.
FIGS. 2 to 6 show a conventional cutting insert and a milling cutter with such a cutting insert mounted thereto.
FIG. 2 is a perspective view of a cutting insert, while FIG. 3 is a side view thereof. The cutting insert 100 comprises a top surface 112, a bottom surface 114 and four side surfaces 116 connecting the top surface 112 and the bottom surface 114. The side surface 116 is connected to the other adjacent side surface 116 through a corner surface 118. Further, a curved auxiliary cutting edge 122 having a radius of curvature R is formed at the intersection between the top and bottom surfaces 112, 114 and the side surface 116. A main cutting edge 124 is formed at the intersection between the top and bottom surfaces 112, 114 and the corner surface 118. The main cutting edge 124 connects an auxiliary cutting edge 122 to an adjacent auxiliary cutting edge 122. The top surface 112 and the bottom surface 114 include a mounting surface 126 and a rake surface 128. The rake surface 128 extends at an inclined angle “a” with respect to the mounting surface 126.
FIG. 4 is a view taken from the corner surface 118 (i.e., in a direction indicated by A). As shown in FIG. 4, the main cutting edge 124 of the conventional cutting insert 100 extends horizontally.
FIG. 5 shows a state where a milling cutter with a conventional cutting insert mounted thereto cuts a surface of a workpiece. FIG. 6 is an enlarged view of a part B of FIG. 5. As shown in FIG. 5, the main cutting edge 124 of the cutting insert 100 performs the function of initially cutting a workpiece along the feed direction of the milling cutter, while the auxiliary cutting edge 122 performs a precision cutting to smoothly polish the portion of the workpiece cut by the main cutting edge 124.
However, due to the manufacturing tolerance of the milling cutter and the cutting insert 100, as well as the mounting error caused when mounting the cutting insert 100 onto the milling cutter, differences in the height of the cutting edges are generated. In order to make the processed surface flat with a high degree of precision, the influence of the error needs to be minimized. As such, the auxiliary cutting edge 122 is formed in the shape of a convex curve having a curvature radius R, which ranges from 100 mm to 200 mm.
Further, the cutting performance of the main cutting edge 124 is closely associated with the axial rake angle of the main cutting edge 124. The axial rake angle represents an angle at which the main cutting edge 124 is inclined relative to the rotation axis of the milling cutter. As shown in FIG. 6, a relief angle is formed to avoid any surface contact between the side surface 116 of the cutting insert 100 and the workpiece. Such an axial rake angle can have a positive value, 0 or a negative value. The positive value represents a case wherein the cutting insert is inclined opposite to the rotational direction of the milling cutter. The negative value represents a case wherein the cutting insert is inclined towards the rotational direction of the milling cutter. The value of 0 represents a case wherein the cutting insert is not inclined relative to the rotational axis. When the axial rake angle has a positive value that becomes greater, chips are more easily eliminated and the cutting resistance is reduced. However, in a conventional cutting insert 100 as shown in FIG. 6, the axial rake angle of the main cutting edge 124 has a negative value (−5° in FIG. 6) to maintain a relief angle generally in the range of 4° to 6°.
Accordingly, since the cutting resistance is significant in the main cutting edge 124 portion, the cutting performance is poor, thereby causing other problems. In particular, when a depth of cut exceeds about 0.5 mm, due to an increase in the cutting resistance, burrs are produced on the processed surface of the workpiece or the main cutting edge 124 applies an excessive pressure to the processed surface of the workpiece, thereby deforming or breaking the structure. Because of these reasons, the depth of a cut is remarkably limited when the conventional cutting insert is used for a precision cutting, which requires a very small surface roughness.